1. Field of the Invention
The present invention relates to a sensor control apparatus for use with a controlled object formed by a sensor element having a solid electrolytic layer and capable of detecting a specific component concentration in a measured gas over a wide range.
2. Description of the Related Art
In the field of automobile engines, for example, a limit-current type oxygen concentration sensor (also called “air-fuel ratio sensor” referred to as “A/F sensor”), which detects the oxygen concentration in an exhaust gas as a detection object, has already been put into practical use. The oxygen concentration sensor is constructed to have a solid electrolytic layer such as zirconia and outputs an electric current signal corresponding to the oxygen concentration in the exhaust gas whenever a voltage is applied a sensor element.
To accurately detect the oxygen concentration, it is necessary to adequately control the voltage applied to the oxygen concentration sensor (the voltage being hereinafter referred to, for brevity, as “applied voltage”). As for such applied voltage control, various techniques have been proposed. According to one example of such prior proposals, which is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2000-081413, the gas concentration detecting apparatus includes a current measurement resistance connected to a sensor element so that an electric current signal measured by the current measurement resistance is fed back to perform the control of the applied voltage. Furthermore, in order to suppress oscillation of the applied voltage, the inclination of the applied voltage characteristic curve is set to be smaller than the AC impedance of the sensor element.
However, in the sensor element, due to individual difference, deterioration with age or the like, variation in sensor capacitance may occur, which can vary the gain in the applied voltage control. In this instance, if a desired gain cannot be realized due to such gain variation, the oscillation of the applied voltage may occur. Thus, there is still room for improvement.